Electronics technology has proliferated with the arrival of inexpensive and high performance integrated circuits, including communications transceivers, microprocessors, and memory. As a result, complex electronics systems, such as personal computers and cellular telephones, have become widely held consumer goods.
Integrated circuits are often fabricated with multiple devices, such as transistors. To minimize integrated circuit size, and hence integrated circuit cost, these devices are often positioned in close proximity to one another. As a result, undesirable inter-device effects can arise. For example, undesired current can leak between devices. Alternatively, a device, such as a transistor, can switch on as a result of positive feedback between proximate devices. This effect is known as latch-up. Leakage current and latch-up are understood by one skilled in the art.
To diminish these unwanted inter-device effects, it is desirable to adequately isolate proximate devices. Conventionally, inter-device isolation is accomplished by creating a field oxide between the devices. The field oxide is an electrical insulator. Thus, proximate devices are substantially electrically isolated if the field oxide has adequate dimensions, including height, length and width. However, the use of a field oxide with inadequate dimensions may result in leakage current and latch-up.
Another undesirable effect is the unintentional fabrication of a parasitic metal oxide semiconductor field effect transistor (MOSFET) 10 on an integrated circuit due to close proximity of inadequately isolated adjacent devices. For example, FIG. 1 illustrates a parasitic MOSFET formed by a conductor 12, active areas 14, and a first oxide 16. The integrated circuit may be a flash memory, including flash cells and MOSFETs, or other semiconductor devices. The first oxide 16 may be field oxide. The active areas may be the sources or drains of flash cells or MOSFETs. The conductor 12 may be a wordline of the flash memory. The design and operation of flash memory are known by one skilled in the art.
The parasitic MOSFET 10 operation is now described. The conductor 12 and the first oxide 16 function as a gate. The active areas 14 operate as sources and drains. Although not constructed as a conventional transistor, the parasitic MOSFET 10 nevertheless may function like one if the first oxide 16 has insufficiently large dimensions. As a result, undesirable leakage current and latch-up may occur. Therefore, it is necessary to maintain adequate first oxide 16 dimensions.
Although adequate first oxide dimensions may be initially fabricated, subsequent semiconductor processing steps tend to greatly diminish the dimensions of the first oxide 16 resulting in poor isolation between devices. Therefore, there is a need in the art for a way of fabricating integrated circuit devices which preserves the field oxide grown for isolation during subsequent processing steps.